Nickel Cadmium (NiCd) batteries have been widely used for more than two decades as rechargeable power sources for portable devices. NiCd batteries, however, can cause environmental pollution, if improperly disposed of, since Cd is a highly poisonous material. NiMHi batteries, on the other hand, provide more power, are free of "memory effect" and are environment-friendly. But these batteries are more tricky and dangerous to charge and discharge than NiCd batteries due to the potential danger of overheating, subsequent venting of hydrogen and then catching fire. Therefore, "smart" or "intelligent" battery packs have gained more and more popularity in portable equipment, especially in laptop computer systems. Unfortunately, these conventional battery packs have not been introduced to small portable equipment such as cellular telephones because they are complex systems more suitable for higher power systems, like laptop and notebook computers, where the protected battery pack's own power consumption, size and ability to communicate with the computer are appropriate.
A serious problem exists today among those subscribers to cellular service who use portable phones: diminished battery life on their portable cell phones due to improper charging. The problem impacts the subscriber because he or she cannot use the portable phone to the extent desired without lugging along one or more spare battery packs. Also, many subscribers turn off their phones between calls to "save the battery", resulting in missed calls.
The problem impacts the cellular service provider also, in that the subscribers will use fewer "minutes" of air-time because of the problem with their batteries. Also, because many subscribers turn off their phones between calls to "save the battery", air-time revenue is lost which otherwise would have been generated by incoming calls to the subscriber. This is actually a more serious problem than the former, because air-time is used anyway trying to establish the call, but no billable call occurs.
It has been shown statistically that users of portable phones make 80 second calls on the average, while users of mobile phones make 180 second calls on the average, a significant lessening of billable air-time. This is caused, of course, by the subscriber's fear of using up his or her battery mid-call.
The manufacturers of portable cellular phones have traditionally incorporated a temperature sensing device, typically a thermistor, into their battery packs. The thermistor signals the associated charger that the battery cells are quite hot. At that point, the charger switches itself off or to a safe trickle current, the assumption being that reaching a predetermined high temperature of the battery cells means they are completely charged. Some chargers incorporate a voltage sensing circuit to determine the point of complete charge, although this is more difficult because of mismatched cells and performance changes over time, and is extremely difficult in a vehicle because of noise on the power feed. Some chargers incorporate both forms of protection.
Although these approaches may be appropriate for nickel cadmium (NiCd) cells, which are better able to tolerate abuse, they are insufficient for Nickel Metal Hydride (NiMHi) cells. As noted above, NiMHi cells vent explosive hydrogen gas when overheated, as can occur during overcharging. Also, venting of electrolyte, which occurs on both cell chemistries when severely overcharged, dramatically reduces remaining cell capacity.
Many manufacturers of chargers for battery packs do not make use of the battery pack thermistor. This is especially prevalent with charger-cords designed to be used in the vehicle's cigar lighter socket. It is not atypical for battery packs to deform from the heat caused by being left in a car overnight, as attached to a charger-cord. Other chargers, such as car-kits and overnight desk chargers, provide an intermediate level of current in an attempt to keep from overheating the cells. Depending on the environmental conditions to which the cells are subjected, the cells can be abused, especially NiMHi. Among those chargers which do take advantage of the battery pack thermistor, the failure mode often is to overcharge.
The state of the art is to focus on charger-based techniques of determining the end of the charge cycle using dT/dt, -dV/dt, etc. The focus of the inventors was instead to discover a way of assuring the subscriber the maximum use of his or her battery pack. The inventors looked for ways of building that assurance into the battery pack itself, so that the pack could be protected against abuse from any charger of any type, whether in good repair or not. Since the charge and discharge contacts are the same on most manufacturer's battery packs, and since packs would be charged while the phone was in use, any "charge" control circuit applied to the pack would have to simultaneously maintain a low-loss "discharge" path.
When a cellular subscriber purchases a brand-new battery today, one that should last for 180 minutes of talking between charges, the first thing he does on the way home is plug the pack into his car charger cord. The charger cord applies full charge current to the pack, whether the pack is charged or not. Later, at home, the subscriber drops the new pack in his desk charger because everyone knows one should charge a battery before one uses it. Later in the day he may drop the unit in his car kit, where it will be further overcharged. By the end of the first day he may have lost half of the battery's initial capacity.
The typical problems encountered by cellular users can be summarized as follows:
1. The chargers "cook" the batteries. PA1 2. Many types of chargers, good and bad, are in use. The consumer may accidentally charge a NiMHi pack using a wrong charger, such as a NiCd charger, or a charger of poor design. PA1 3. Some batteries, although equipped with a "gas-gauge" capacity indicator, can only measure the remaining power and cannot terminate charging when appropriate.
U.S. Pat. No. 5,218,284 (Burns et. al.) discloses a "universal battery" which embeds a switch matrix and switching power supply circuit to regulate the charge and discharge. It is a very complicated system, and its switching power supply converter has a conversion power loss. The useful efficiency is about 80 to 90%. Additionally, in order to block reverse current, a diode is used at the output which drops the output voltage, reducing the efficiency further.
Due to the limitations mentioned above, the prior art has not realized a NiMHi battery system with proper overcharge protection. A need therefore continues to exist for a low-cost NiMHi embedded charge control system for a cellular battery pack.
In recent years, many efforts have been made to develop battery protection circuits but almost all the work has been focused on chargers, not on the battery pack itself. Since the end user still may still mix the use of chargers and batteries, the prior art has not provided a workable solution to the problems noted above.